Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

38 ENGINEERING PROBLEMS OF PANAMA CANAL. (4) much of the rock traversed by isthmian streams is soft volcanic tuff and agglomerate that is easily converted into fine sediment. This forms a large proportion of the materia] carried by the streams during flood time, and in the lower rivers it covers and obscures much of the gravel that might otherwise bo available. STABILITY OF CANAL FOUNDATIONS. GEOLOGIC STRUCTURE OF FOUNDATION ROCKS. GENERAL CONSIDERATIONS. Geologic conditions surrounding tho foundations of heavy struc- tures largely determine the plan, cost, and efficiency of such founda- tions. For instance, if a dam is built on a foundation rock of lime- stone leakage may occur, and that leakage if not remedied is likely to increase, because limestone is slightly soluble in ground water. If the water comes from a surface where there is much decaying vegetation, its solvent power is greatly increased by the increased content of organic acids and carbon dioxide.. Wherever such water can penetrate a crevice in limestone, it will slowly enlarge that crevice by solution. Consequently, most large caves occur in limestone. The limestone formations on the Canal Zone locally con- tain many caves, openings, and irregular pitted surfaces, because the decay of the luxuriant vegetation there gives the ground water a relatively high content of organic acids and greatly increases its power as a solvent. Hence considerable care is necessary in locating a dam on. a limestone foundation rock in order to bo sure that no leaks, which may later enlarge, will occur. If a masonry dam is to bo secured to the foundation rock along its bottom and at both ends, certain geological factors must bo con- sidered. If the rock is shaly, thin bedded, and cut by many joints or fault planes, there is danger that certain blocks or sections of rock, or the upper bods to which the dam is secured, may under great pres- sure slip on the rocks below them and cause the dam to give away. Along many fault planes and along some bedding planes there are smoothened partings that readily lend themselves to slipping. Fur- thermore there is not only the pressure of the water against the wall of tlio dam, tending to push it over, and thus straining to widen the fissures in the foundation rock to which the dam is secured, but thero is also the hydrostatic pressure of tlio water tending to force itself into those fis- sures and bedding pianos and thus to exert a lifting or floating strain on tho dam and on the slabs of rock to which the dam is attached. Another important item relating to dams is to see that excessive leakage does not occur cither through the dam, around the founda- tons, or through tho rim of the dammed-in basin. In order to ascertain what tho percentage of leakage will be, individual tests must be